1. Field of the Disclosure
The present disclosure pertains to logging while drilling apparatus and more particularly to acoustic logging while drilling apparatus and generating and using Stoneley waves to measure formation permeability and electroacoustic constant.
2. Summary of the Related Art
Acoustic wellbore logging instruments are used to measure velocities of earth formations in one or more modes of acoustic energy propagation. Acoustic wellbore logging instruments are typically used in liquid-filled wellbores drilled through the earth formations. Velocity is generally determined using these instrument by measuring the time taken by an acoustic energy pulse to traverse a particular distance along the wall of the wellbore. The wall forms the interface between the liquid in the wellbore and the earth formations.
One form of acoustic energy pulses of particular interest to the invention is referred to as “Stoneley” waves. Stoneley waves, also known as tube waves, are essentially guided pressure pulses travelling in the wellbore. It had been determined in earlier research that a relationship is likely to exist between the transmission properties of Stoneley waves as they propagate along the wellbore, and the hydraulic characteristics of the formations along the wellbore wall. See for example, J. Rosenbaum, Synthetic Microseismograms: Logging in Porous Formations, Geophysics, vol. 39, pp. 14-32, Society of Exploration Geophysicists (1974). Determining formation permeability was not practical using the acoustic logging instruments available at the time the Rosenbaum reference was published because those instruments typically did not generate detectable Stoneley waves, and in the instances where they did so, separation of the Stoneley waves from the acoustic signals as detected was very difficult.
U.S. Pat. No. 5,784,333 to Tang et al., having the same assignee as the present disclosure and the contents of which are incorporated herein by reference, discloses a method for determining the permeability of earth formations penetrated by a wellbore from acoustic signals measured by an acoustic wellbore logging instrument. The method includes separating components from the measured acoustic signals which represent Stoneley waves propagating through the earth formations. Signals representing Stoneley waves propagating through the same earth formations are synthesized. The separated acoustic signal components and the synthesized Stoneley wave signals are compared. The permeability is determined from differences between the synthesized Stoneley wave signals and the separated acoustic signal components. In a preferred embodiment, the step of calculating the permeability includes inversion processing a wave center frequency shift and a wave travel time delay with respect to the permeability of the earth formations.
The present disclosure is directed towards a method and apparatus for directly measuring the formation permeability and the electroacoustic constant of a porous formation. It makes use of the fact that an elastic wave (such as a Stoneley wave) propagating along the borehole wall produces an electrical signal that can be measured.